anyone have trouble playing mkv files on their mac?

i just downloaded some planet earth episodes. they are 720p / mkv files. i was using VLC but i keep getting a "computer too slow" message and the video freezes. i have my macbook pro plugged into my tv with a dvi to hdmi cable. i have a macbook pro (core2duo, 2gigs of ram, 2.16ghz) if that helps.